The scientific study of human twins began in the 1870s when Sir Francis Galton (1822-1911) published a series of articles arguing that heredity (nature) was a stronger factor than environment (nurture) in determining the respective characteristics of twins. He later suggested that identical twins might come from a single egg while non-identical twins might come from two separate eggs, simultaneously fertilized and implanted. This guess, published in Inquiries into Human Faculty and its Development (1883), was later proved correct, but neither Galton nor his contemporaries had any rigorous evidence to support it.

Human twin studies are important for genetic and psychological research because twins provide a natural control for experiments. Because respect for each twin's feelings, privacy, and personhood is easy for even the best-intentioned scientist to compromise, and because twin research readily evokes eugenics, some twin studies will probably always remain controversial, both ethically and scientifically...